Many industrial, medical, marine, transport and aerospace systems with graphical user interfaces (GUIs) require display modules that are rugged, robust and capable of consistent, reliable operation, writes David Potts is divisional marketing manager at Anglia.
The user interface needs to be capable of displaying bright, vivid display images and information.
The front panel will have a significant impact on user perception: is the display image bright and colourful? Is the information presented on the display easy to view in all application environments? Extreme environments and harsh conditions can present operating and implementation challenges for displays used in electronics systems and equipment.
Designers need to ensure that displays provide excellent optical performance and can overcome any demands presented by ambient light conditions, temperature extremes and mechanical shock and vibration. Many display manufacturers are primarily driven to develop displays for high volume OEM markets and applications such as smartphones, tablet computers and consumer entertainment such as media players and televisions.
The production volumes associated with these markets will drive display development and the adoption and implementation of new technologies.
The latest display technologies used in high volume consumer market sectors will, over time, migrate to high reliability, rugged displays. An increasingly small number of display manufacturers are actively investing in and developing displays specifically for applications requiring improved optical performance, high reliability and product longevity. For example, Hitachi has recently introduced a Rugged+ product range directly addressing the requirements of applications needing a robust and reliable TFT display.
Many user interfaces require highly consistent optical performance and accurate image reproduction. Demand for wide viewing angle and wide field of vision displays has increased in high reliability systems. Contrast and colour characteristics must be maintained over a wide range of viewing angles and positions.
Viewed from an acute angle, a conventional TFT display can exhibit a loss of brightness which can result in a reduction in contrast and the display image inversion. Wide viewing angle technologies such as in-plane switching (IPS) and vertical alignment (VA) are now extensively used in the manufacture of smartphones, monitors and televisions.
In-plane switching and vertical alignment TFT display modules overcome the limitations of conventional TFT display technology by providing a stable colour image from any viewing angle (up to 176° in both horizontal and vertical planes), excellent colour saturation, colour stability and high contrast ratios.
High reliability markets now benefit from the features of in-plane switching technology as ruggedised IPS displays become more widely available. The evolution of rugged, robust IPS displays offer a very effective ‘cost versus performance’ display solution when compared with traditional TN (twisted nematic) TFT display modules. Rugged in-plane switching (IPS) displays offer good colour saturation with very high contrast ratios. Good black levels combined with improved transmittance provide contrast ratios of up to 1,000:1.
The optical performance or readability of a display is defined by its brightness and contrast specifications. At present the benchmark brightness specification for a rugged LCD display is 600cd/m². For sunlight-readable applications this increases to 1,000cd/m². This enhanced brightness level can be achieved by simple improvements in the efficiency of the optical stack without any increase in power consumption.
White LED backlights have all but replaced Cold Cathode Fluorescent Lamp (CCFL) backlights in rugged LCD display modules. LED backlights are more robust and reliable than the CCFL backlights, with the advantage of not needing a high voltage DC-AC inverter to drive the backlight. Some rugged LCD displays also feature on-board LED controllers, enabling 0% to 100% backlight dimming, whereas CCFL backlight dimming was generally only possible to 50%.
LED lifetime characteristics are important, with lifetime estimations of between 50,000 to 100,000 hours being quoted by display manufacturers. Scrutiny of manufacturer’s data sheets is necessary to determine exactly how the LED lifetime is being specified.
Display manufacturers will often use the same LEDs in backlight units and the difference between a specified 100,000 hour or 50,000 hour lifetime may be the result of the brightness value used as an initial reference point. The LED backlight lifetime is less affected by operating temperature which is essential for ensuring a reliable, maintenance-free display installation in a high reliability application.
The physical characteristics of the display are important and the mechanical design of a rugged LCD display is likely to differ significantly from a display used in a high volume OEM application.
A high reliability, rugged display needs to be suitable for use in many varied applications. Ease of mounting the display securely within a mechanical housing or fixture is paramount. It should have mounting points that are easy to use and do not require any unique mounting fixture to be designed. The internal construction of a rugged display is specifically designed and tested to ensure shock and vibration in a harsh and extreme application environment can be endured.
The mechanical size and aspect ratio of the display is likely to be determined by the system design needs. ‘Classic’ 4:3 aspect ratio displays with screen sizes of 5.7in, 6.5in, 10.4in and 12.1in are all commonly available. Wide format 15:9 aspect ratio with screen sizes of 7.0in, 8.0in and 9.0in are being widely adopted, enabling information, data and graphics to be easily represented and displayed. Many high-reliability applications will use a dedicated embedded hardware or a single board computer (SBC) for the core application task and the ability to drive an LCD display. Many integrated CPUs and SBCs integrate high levels of functionality, some allowing backlight control, and will drive display resolutions of QVGA (320x240) and above.
There are two industry standards for the electrical interface used on rugged LCD displays: RGB CMOS and LVDS. These two data interfaces are implemented to use either 6-bit or 8-bit data which enables a colour palette of up to 262k and 16.7 million colours respectively. The implementation of LVDS display interfaces on SBC and rugged LCD display modules is becoming mandatory as display resolutions increase and the ability to provide remote presentation of information is required.
A high reliability applications suggests that the environment in which a product or system is going to be used will be harsher and more extreme. A navigation panel on a marine craft, an operator panel on a forklift truck or a portable medical diagnostic monitor will almost certainly present a testing user environment.
Many consumer products with user interface displays will have an operating temperature specifications of +5°C to +40°C. This typically relates to a stable domestic or office environment. The operation of the display outside this range will not be guaranteed by the display manufacturer. The rugged display features the use of wide temperature liquid crystal, materials and components to provide a guaranteed operating temperature range of -30°C to +80°C.
A few display manufacturers have recognised the importance of the market for high reliability displays and they are optimising the specifications, integration and interfaces of the rugged LCD display. The migration of wide viewing angle technologies coupled with the use of extended temperature range components and dedicated mechanical design now enables the system designer to choose a display from an expanding portfolio of rugged TFT display solutions. By using a new generation rugged TFT display module, market leading optical performance, long term availability and continuity of supply can be ensured.
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